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CHAPTER 1

INTRODUCTION

1.1 GENERAL

Severity of ground shaking at a given location during an earthquake

can be minor, moderate and strong. Relatively speaking, minor shaking

occurs frequently; moderate shaking occasionally and strong shaking rarely.

The engineers do not attempt to make an earthquake proof building

that will not get damaged even during the rare but strong earthquake; such

buildings will be too robust and also expensive. Instead, the engineering

intention is to make buildings earthquake resistant; such buildings resist the

effects of ground shaking, although they may get damaged severely but would

not collapse during the strong earthquake. Thus, safety of people and contents

is assured in earthquake-resistant buildings, and thereby a disaster is avoided.

This is a major objective of seismic design codes throughout the world.

1.2 EARTHQUAKE DESIGN PHILOSOPHY

The earthquake design philosophy may be summarized as follows.

a) Under minor but frequent shaking, the main members of the

building that carry vertical and horizontal forces should not be

damaged; however building parts that do not carry load may

sustain repairable damage.

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b) Under moderate but occasional shaking, the main members

may sustain repairable damage, while the other parts of the

building may be damaged such that they may even have to be

replaced after the earthquake.

c) Under strong but rare shaking, the main members may sustain

severe (even irreparable) damage, but the building should not

collapse. Thus, after minor shaking, the building will be fully

operational within a short time and the repair costs will be

low. And, after moderate shaking, the building will be

operational once the repair and strengthening of the damaged

main members is completed. But, after a strong earthquake,

the building may become not functional for further use, but

will stand so that people can be evacuated and their property

recovered.

The consequences of damage have to be kept in view in the design

philosophy. For example, important buildings, like hospitals and fire stations,

play a crucial role in post-earthquake activities and must remain functional

immediately after the earthquake. These structures must sustain very little

damage and should be designed for a higher level of earthquake protection.

The collapse of dams during earthquakes can cause flooding in the

downstream reaches, which itself could lead to a secondary disaster.

Therefore, dams (and similarly, nuclear power plants) should be designed for

a still higher level of earthquake motion. The performance of the building

during earthquake is shown in Figure 1.1.

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Figure 1.1 Performance of the Building (Murthy 2002a)

1.3 EARTHQUAKE OCCURANCE IN COIMBATORE

Until 1967, most of the engineers and scientists in the country were

under the impression that peninsular India was free from seismic activity. But

the earthquakes at Koyna (1967) and Latur (1993) changed their perception.

Tamil Nadu is thought to be mostly free from seismic activity. But an

earthquake rocked Coimbatore as far back as 1900. A study of the earthquake

will help in understanding its nature and drawing lessons that would be of

help in the event of its recurrence.

An earthquake of moderate magnitude (6 on the Richter scale)

occurred near Coimbatore at 3.11a.m. (Indian Standard Time) on February 8,

1900. Its maximum intensity was VII on the Modified Mercalli Intensity

Scale. It caused the largest extent of damage at two locations - Coimbatore

and Coonoor - and its impact was felt in the areas that lie between Udipi in

the north and Thiruvananthapuram in the south and Kozhikode, Bangalore,

Chennai, Nagapattinam and Madurai in the east-west direction. The epicenter

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of the earthquake was located at 10° 45' North Latitude and at 76° 45' East

Longitude.

At Coimbatore (Intensity VII), several buildings were seriously

damaged. Roof tiles collapsed. The central prison building suffered the most.

A Roman Catholic Church near the railway station gave in. A boy who was

trapped in a collapsed mud house was rescued. At Coonoor (Intensity VII),

the railway refreshment room and the Commissioner's bungalow developed

cracks. The Nilgiri Railway (Intensity VII) suffered losses owing to the fall of

boulders on the track. The shock was felt at Kuppam (Intensity VI) and

Perundurai (Intensity VI). Records from Mysore (Intensity VI) said that the

resting cattle stood up. The roof tiles of most of the houses were damaged and

walls developed cracks.

A statistical analysis indicates that such seismological events might

recur once in 100 years, plus or minus about 30 years. Although such

statistical forecasts are probabilistic and somewhat gross, this gives a

reasonable idea about the seismogenic potential of the region. It is recorded

that seismic activity in this region has been on the rise for the past 15 years.

There were two earthquakes, each with a magnitude of around 5.0 on the

Richter scale, in Idukki and Coimbatore districts on Kerala-Tamil Nadu

border in December 2000 and January 2001 respectively. It is also recorded

that seismic activity began sometime in 1988 when the Idukki area was

shaken by an earthquake of a magnitude of 4.8 on the Richter scale.

Incidentally, the Idukki dam happens to be located in this seismically

vulnerable area, the dam area has a seismic network.

Modern seismological instruments record earthquakes of a very

low magnitude - up to -2.0 or so. As a result of the tremendous increase in

their detection potential, instruments record thousands of seismic events of

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small magnitudes, known as micro-earthquakes or ultra-micro-earthquakes

(of negative magnitudes of up to -3.0). Seismologically speaking, it is not

correct to describe them as earthquakes. These are minor geological

movements that occur routinely.

The Governments of Tamil Nadu and Kerala should use this

historical information as inputs to protect the people of their states from any

seismic event in future. Earthquakes do not kill people. It is the collapse of

man-made structures that kills.

Studies indicate that any seismic event in the Coimbatore region

could reach a maximum magnitude of 5.5 to 5.75 on the Richter scale. The

attempt here is to awaken the people and the local body administration so that

the people are trained and prepared to face any such eventuality. This is a

probabilistic assessment, done with a view to increasing the people's

preparedness.

Earthquakes are part of the dynamic movement of the earth. All the

advancements in Science and Technology cannot prevent an earthquake.

People should learn to live with this reality. Compared to other disasters, an

earthquake lasts for the least duration and the possible response time is very

low. Even if one gets only 10 seconds, one should come out of his or her

house when an earthquake occurs.

Recently on 11th April 2012, a mild earthquake shook Coimbatore

city during afternoon around 2.30 p.m. following earthquakes in Indonesia to

a magnitude of 8.9 on Richter scale. People living near Saravanmpatti of

Coimbatore district felt the tremors and shake of building and ran to the

safety zones to safeguard their lives. Such tremors and shakes were felt in

Chennai, some part of Tamilnadu and East coast of India on the same day.

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During earthquake many people were killed and badly injured

because of

1) Poorly constructed buildings either totally or partially

collapsed.

2) Walls collapsing within narrow streets, burying people

escaping into them.

3) Untied roofs and cantilevers falling onto the people.

4) Free standing high boundary walls, parapets and balconies

falling due to severe shaking.

5) The failure of modern reinforced structures with large open

spaces at ground to first floor level,

a. For example garage or shop spaces, collapsing and

burying occupants (soft storey collapses)

6) Inhabitants not knowing how to respond to the shaking and

collapse of walls around them.

1.4 PRESENT SCENARIO OF STRUCTURES IN

COIMBATORE

In Coimbatore many of the existing buildings were built with the

masonry infill as nonstructural element and the analysis as well as design is

carried out by using only the mass but neglecting the strength and stiffness

contribution of infill. Therefore, the entire lateral load is assumed to be

resisted by the frame only. One of the disadvantages of neglecting the effect

of infill is that, the building can have both horizontal as well as vertical

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irregularities due to uncertain position of infill and openings in them. Also,

the infill walls are sometimes rearranged to suit the changing functional needs

of the occupants. The changes are carried out without considering their

adverse effects on the overall structural behavior.

In Coimbatore, the structural designer’s during past years used to

ignore the stiffness and strength of infill in the design process and treated the

infill as non-structural elements. This is mainly due to lack of generally

accepted seismic design methodology in the present Indian Code that

incorporated structural effects of infill. Hence, there is a vital need to create

awareness to develop a design methodology for seismic design of masonry

infill Reinforced Concrete structure.

1.5 METHODS OF ANALYSIS

For seismic performance evaluation, a structural analysis of the

mathematical model of the structure is required to determine force and

displacement demands in various components of the structure. Several

analysis methods, both elastic and inelastic, are available to predict the

seismic performance of the structures.

1.5.1 Elastic Methods of Analysis

Sermin Oguz (2005), the force demand on each component of the

structure is obtained and compared with available capacities by performing an

elastic analysis. Elastic analysis methods include code static lateral force

procedure, code dynamic procedure and elastic procedure using demand-

capacity ratios. These methods are also known as force-based procedures

which assume that structures respond elastically to earthquakes.

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In code static lateral force procedure, a static analysis is performed

by subjecting the structure to lateral forces obtained by scaling down the

smoothened soil-dependent elastic response spectrum by a structural system

dependent on force reduction factor, "R". In this approach, it is assumed that

the actual strength of structure is higher than the design strength and the

structure is able to dissipate energy through yielding.

In code dynamic procedure, the force demands on various

components are determined by an elastic dynamic analysis. The dynamic

analysis may be either a response spectrum analysis or an elastic time history

analysis. Sufficient number of modes must be considered to have a mass

participation of at least 90% for response spectrum analysis. Any effect of

higher modes is automatically included in time history analysis.

In Demand and Capacity Ratio (DCR) procedure, the force actions

are compared to corresponding capacities as demand and capacity ratios.

Demands for DCR calculations must include gravity effects. While code

static lateral force and code dynamic procedures reduce the full earthquake

demand by an R-factor, the DCR approach takes the full earthquake demand

without reduction and adds it to the gravity demands. DCRs approaching 1.0

(or higher) may indicate potential deficiencies.

Although force-based procedures are well known to engineering

profession and easy to apply, they have certain drawbacks. Structural

components are evaluated for serviceability in the elastic range of strength

and deformation. Post-elastic behavior of structures could not be identified by

an elastic analysis. However, the post-elastic behavior should be considered,

as almost all structures are expected to deform in inelastic range during a

strong earthquake. The seismic force reduction factor "R" is utilized to

account for inelastic behavior indirectly by reducing elastic forces to inelastic.

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Force reduction factor, "R", is assigned considering only the type of lateral

system in most codes, but it has been shown that this factor is a function of

the period and ductility ratio of the structure as well.

Elastic methods can predict elastic capacity of the structure and

indicate where the first yielding will occur, however they don’t predict failure

mechanisms and account for the redistribution of forces that will take place as

the yielding progresses. Real deficiencies present in the structure could be

missed. Moreover, force-based methods primarily provide life safety but they

can’t provide damage limitation and easy repair.

The drawbacks of force-based procedures and the dependence of

damage on deformation may have led the researchers to develop

displacement-based procedures for seismic performance evaluation.

Displacement-based procedures are mainly based on inelastic deformations

rather than elastic forces and use nonlinear analysis procedures considering

seismic demands and available capacities explicitly.

1.5.2 Inelastic Methods of Analysis

Structures suffer significant inelastic deformation under a strong

earthquake and dynamic characteristics of the structure change with time. So

investigation of the performance of a structure requires inelastic analytical

procedures accounting for these features. Inelastic analytical procedures help

to understand the actual behavior of structures by identifying failure modes

and the potential for progressive collapse. Inelastic analysis procedures

basically include inelastic time history analysis and inelastic static analysis

which is also known as pushover analysis.

The inelastic time history analysis is the most accurate method to

predict the force and deformation demands at various components of the

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structure. However, the use of inelastic time history analysis is limited

because dynamic response is very sensitive to modeling and ground motion

characteristics. It requires proper modeling of cyclic load deformation

characteristics considering deterioration properties of all important

components. Also, it requires availability of a set of representative ground

motion records that accounts for uncertainties and differences in severity,

frequency and duration characteristics. Moreover, computation time, time

required for input preparation and interpreting voluminous output make the

use of inelastic time history analysis impractical for seismic performance

evaluation.

Inelastic static analysis or pushover analysis is the preferred

method for seismic performance evaluation due to its simplicity. It is a static

analysis that directly incorporates nonlinear material characteristics.

1.6 OBJECTIVES, SCOPE AND METHODOLOGY OF THE

STUDY

Objectives

This study aims to investigate the effect of brick masonry infill wall

on a reinforced concrete moment resisting frame conventionally designed as a

bare frame, using available macro model .The specific objectives of the study

are:

i. To study the effect of brick masonry infill wall on existing

reinforced concrete moment resisting frame, subjected to

earthquake induced by the lateral load.

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ii. To study the effect of an existing reinforced concrete moment

resisting bare frame, subjected to earthquake induced by the

lateral load.

iii. To study the performance of the structural beam element

retrofitted with Glass Fiber Reinforced Polymer (GFRP)

composites.

Scope

i. This study deals with the reinforced concrete moment

resisting frame with full unreinforced brick masonry infill

frame and bare frame subjected to earthquake forces.

ii. Infill and bare frame are analysed by the Nonlinear Analysis

(Pushover Analysis) method using SAP 2000 software

version11.

iii. The performance levels of various components of buildings,

behavior of the components and failure mechanism in

buildings, hinge formation, performance point, base shear

versus roof displacement and capacity curve will be studied in

detail.

iv. The weak Reinforced Cement Concrete (RCC) structural

elements are identified and retrofitted with Glass Fiber

Reinforced Polymer (GFRP) composites subjected to static

loading.

v. The performances of the existing buildings are improved by

local retrofitting technique based on SAP 2000 results.

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vi. To suggest a suitable retrofitting technique so that the strength

and performance level of deficient structure could be

improved during the earthquakes.

Methodology

i. To identify the typical deficiency existing in reinforced

concrete building and collection of relevant structural data.

ii. To evaluate the structural diagnosis using Nondestructive

technique facilities.

iii. Modeling of the structural system and Analysis of the model

for seismic vulnerability by using SAP 2000 version 11.

iv. Modeling and experimental studies on the retrofitted deficient

members.

v. The performances of the existing buildings that are improved

by local retrofitting technique based on SAP 2000 results.

vi. To suggest a suitable retrofitting techniques so that the

strength of structure could be enhanced and performance of

structure during an earthquake could be improved.

1.7 ORGANIZATION OF THE THESIS

This thesis consists of eight chapters. The chapters are arranged in

the following order so as to present the study carried out in a coherent manner

in this thesis.

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Chapter 1 deals with the general introduction about the earthquake

concepts, objectives, scope and methodology of the present investigation.

Also includes the impact of this study in the present scenario in India.

Chapter 2 reviews the research works carried out during the last

few decades on analytical and experimental studies on pushover analysis and

retrofitting of the reinforced concrete structures.

Chapter 3 discusses the details of the existing building under

study, modeling aspects were considered and the procedure for seismic

evaluation of buildings using pushover analysis by SAP 2000 version 11.

Chapter 4 presents the results and discussions based on the

analytical results and its evaluation on existing RCC building through

pushover curve, capacity curve and hinge formation.

Chapter 5 presents the seismic retrofitting method on existing

RCC bare frame building, based on experimental data pertaining to identified

GFRP composite beams.

Chapter 6 presents the analysis of bare frame with strengthened

beams by SAP 2000. The improved performance level of existing RCC

building is discussed in detail. It is compared with and without retrofitting of

the RCC structures.

Chapter 7 outlines of the General conclusions and specific

conclusions based on analytical and experimental work.

Chapter 8 gives suggestions based on the research work and

recommendations for future research are discussed.